Explosion-proof,horn-loaded loudspeaker



United States Patent Haskel A. Blair Oklahoma City, Oklahoma 635,148

May 1, 1967 Oct. 27, 19 70 LTV Ling Altec., Inc. Oklahoma City, Oklahoma a corporation of Delaware Inventor Appl. No. Filed Patented Assignee EXPLOSION-PROOF, HORN-LOADED LOUDSPEAKER 6 Claims,4 Drawing Figs.

U.S. Cl 181/31, l79/l'15.5(BS), 179/184, 181/27 lnt.C1 ..G10k 11/10 FieldofSearch 179/115(BS).

115(H).190: 181/311Al; l/27 [56] References Cited FOREIGN PATENTS 1,005,560 4/1957 Germany 179]] 15.5 1,058,556 6/1959 Germany 179/] 15.5

Primary Examiner-Kathleen 1-1. Claffy Assistant Examiner-Thomas L Kundert Attorney-Dunlap, Laney, Hessin & Dougherty Patented 0a.;27, '1970 +26 FIG 3 lj gzhw j r 2/4 W\\ 8 EM g I m v 2 'l -l\ W l\ W f v a 3 l V 14/! I I I00 200 500 I000 5000 20000 v FREQUENCY m CYCLES PER SECOND v H6 3 FIG 4 f HASKEL'A. BLAIR;

' INVENTOR BYYM.

AGENT.

EXPLOSION-PROOF, HORN-IDADED LOUDSPEAKFR This invention relates to loudspeakers and more particularly to a loudspeaker having a horn in combination with a driver unit.

For many years, horn-loaded loudspeakers have generally been well respected for their efficiency and fidelity in reproducing sound at selected frequencies. Since horns are inherently high-pass" filters, their high frequency range is generally unlimited, while their low frequency range is determined within certain limits by relatively well know techniques in their design. Accordingly, a driver unit intended for use with a particular horn preferably is itself designed to complement the horn design, so that the driver unit will not expand energy in operating in a frequency range that the horn cannot accommodate. For this reason, driver units for horn systems are almost always of the compression type,'with the back of the unit being completely sealed to provide a stiff air cushion for the back of the diaphragm. The stiffness imparted to the diaphragm by the air cushion raises the resonance of the unit and makes it more efficient in those high frequency ranges where the horn is inherently most efficient,

Compression "driver units have also been frequently. employed where their sealed housings have had other beneficial effects. For example, when it is desired to make any loudspeaker explosion proof, that'portion of loudspeaker which houses the voice coil, etc., is usually the only place where protective steps can be taken with any assurance of effectiveness. This is because the electrical conductors in the'voice coil, the connections between the coil and the lead-in wires, etc., are the only really sensitive items in a loudspeaker, and because a break in one of the electrical conductors could cause a spark which in turn would cause an explosion if an explosive gas or mixture were present. Protection against this risk in a compression driver is normally obtained at least partially by enclosing the entire driver unit (including all electrical connections) in a relatively heavy protective housing, such that any explosion that might possibly occur around the driver unit will be contained arid not allowed to set'off a large explosion in the ambient environment. It should be mentioned, perhaps. that the accepted criterion for an explosion-proof device is not that it will not itself explode, but that the device will not in turn set off a general explosion in its environment if it does experience an internal explosion.

Compression drivers of the prior art which have already had sealed backs, have only lacked a satisfactory way of precluding explosions from being propagated in a frontal direction, i.e., through the sound output aperture, in order to permit the construction of optimum, horn-loaded, explosionproof loudspeakers. By satisfactory. it is meant a truly effective means for preventing the spread of an explosion through the sound output aperture, while at the same time permitting the sound waves to pass therethrough in a relatively uninhibited manner.

Accordingly, it is an object of this invention to provide a means for making an improved explosion-proof loudspeaker.

Another object is to provide a means for inhibiting the spread of an explosion that has originated within a driver unit, while prior to the explosion not unduly inhibiting the normal propagation of sounds from the driver unit.

A further object is to provide a filter means for a compression driver which limits the ingress of environmental gases and gas-borne particles into the driver.

Other objects and advantages will be apparent from the specification and claims and from the drawing illustrative of the invention.

IN THE DRAWING:

FIG. 1 is a partially sectioned, elevation view of a compression-driven, horn-loaded loudspeaker having a folded horn which is characteristic of the prior art;

FIG. 2 is a partially sectioned, elevation view of one embodiment of a loudspeaker made in accordance with the invention;

FIG. 3 is an isometric view of one of the filters shown in FIG. 2; and

HO. 4 is a performance curve showing results obtained with several embodiments of the invention having difierent combinations of filters.

With initial reference to FIG. 1, a loudspeaker assembly 10 is shown which is typical of prior art loudspeakers that have been approved and rated as explosion proof. The driver unit 11 is protected by placing it within a relatively heavy and somewhat bulky housing 12 which isolates the driver from the ambient'environment. Threaded means are employed to connect the driver until 11 with a trumpet or horn 14 through which the sound is propagated. As can be seen from inspection, the driver unit 11 is mounted toward the rear of the assembly l0, and it can be disengaged from the born 14 only by removing at least part of the housing 12 and subsequently moving the driver rearwardly with respect to the horn. The space which is required to permit removal of the driver unit 11 and its housing 12'dictates that the mounting bracket 15 be rather large, especially if replacement of the driver is to be accomplished without disturbing the positionof the horn 14.

With reference to FIG. 2, a substantially round loudspeaker 16 which comprises one embodiment of the invention is shown partially in cross section in order to best compare it with the loudspeakerof FIG. 1. it should be understood, of course, that the size differential of the two FlGS. is not iritended to be'significant; rather, as will be made clear herein, it

' is, among other things, a novel relationship of parts that structurally distinguishes the two loudspeakers and contributes to different results.

As is common whenever it is desired to achieve an exponentially expanding air column which is as long as possible in a relatively small space, a folded horn 17 is employed. While the horn is represented as being round. other shapes such as rectangles, squares, etc., have been employed using the concepts disclosed herein, and it should be understood that it is not intended to limit the invention to round horns. The horn l7 comprises a plurality of annular elements having surfaces that reflect and bend the sound'waves as they pass from a throat 18 to a mouth 19, with the mouth quite naturally defining the front of the loudspeaker 16. A driver unit having a sound output aperture 21 is affixed in some manner such as by threads in a position where the sound output aperture is in communication with the horn throat 18. As shown, the sound output aperture 21 is arranged to propagate sound waves in a substantially rearward direction with respect to the loudspeaker l6. A first or inner element 22 and successive elements 23. 24 and 25 are adapted to guide the sound waves without causing undue distortion through three changes of direction, i.e., from the original substantially rearward direction to a concluding substantially forward direction. The sound channel, then, between the throat l8 and the mouth 19, comprises a serial array of annular segments bounded by the confronting surfaces of the elements or walls 22, 23, 24, 25.

A cover member 26 having a smooth annular surface 27 is adapted to be affixed interiorly of the born 17 where it cooperates with the inner element 22 to enclosure all but the sound output aperture 21 of the driver unit 20. The annular surface 27 is adapted to bear against a confronting annular surface 28 on element 22, and, when held together by bolts or the like, the inner element 22 and the cover plate 26 comprise structure which isolates the driver unit 20 from the ambient environment; thus, it functions in the same manner as does the housing 12 in FIG. 1. The cover 26 is preferably installed in such a manner that it can be later removed in order that the driver unit 20 may be similarly removed, for repair, replacement, etc. In addition to enclosing the driver unit 20, the structure consisting of walls 22, 26 quite naturally also encloses a quantity of air; that is, of the matter contained in the space enclosed by the walls, everything that is not solid is gaseous. The quantity of air will vary, of course, from one embodiment to the next, to the extent of variations in the spacing and size of the wall 22, the cover plate 26, and the driver 20. While the presence of air around the driver unit is of no initial consequence, there exists the definite possibility that an explosive gas will bleed between the surfaces 27, 28 into the enclosure. Thus, after an extended period of time, the gas within the walls 22, 26 must realistically be considered to be potentially explosive. In any event, the walls 22, 26 are made sufficiently strong in relation to the volume of gas enclosed thereby, so that, if the gas were somehow to be ignited and an explosion resulted, the walls will effectively resist rupture.

The ability of the walls 22, 26 to resist rupture from an internal explosion is not ideally the result of wall strength alone. In addition to wall strength, an advantageous feature that contributes to explosion-proof properties is the construction of the interface between surfaces 27, 28 such that it is deliberately not made 100 percent airtight. Hence, if an explosion does somehow occur within the walls 22, 26, a substantial portion of the hot gases resulting from the explosion will vent themselves through the microscopic channels between surfaces 27, 28 and thereby relieve the pressure tending to rupture the walls. The escaping, explosion gases themselves pose no problem because they are cooled to a safe value as they travel the relatively long paths before reaching the ambient gases outside of the walls 22, 26. Most persons will recognize, too, that if the volume of enclosed gas is, for example, decreased. the design strength of the walls 22, 26 can be relaxed by making them thinner or using a weaker material, etc. Conversely, enlarging the quantity of gas will generally increase the strength requirement in order to preclude rupture due to an explosion. It is believed, therefore, that no precise dimensions need be quoted in order to adequately describe the walls 22,

26, since those skilled in the art will understand what is meant when it is said that, in the illustrated embodiment, the structure surrounding the driver unit is explosion proof.

It is worthy of noting at this point that the wall 22 serves a double purpose in that it serves as part of the enclosure for the driver unit 20 as well as part of the horn structure 17. By arranging the loudspeaker components as shown, such that a single wall 22 is common to both the horn l7 and the structure protecting the driver unit 20, at least one wall has been eliminated and a loudspeaker of fewer component parts is provided. A further advantage is that the driver unit 20, which (because of its electrical conductors and connections) is the most sensitive part of a loudspeaker, is safely disposed interiorly of the horn 17 where, for example, it is less susceptible to impact from falling objects. Furthermore, the means for mounting the element 22 and the driver unit 20 within the horn 17 can be made more rigid than is sometimes possible with a single threaded means which is common in the prior art. As shown in FIG. 2, two of the plurality of mounting bolts 29 are widely spaced to impart stability to the internal elements of the loudspeaker l6.

Placed across the throat 18 of the horn 17 to close the same are a pair of screens or filters 30 separated by a washer 31 and held in position by a plug 32 which threadably engages the inner element 22. The washer 31 has an aperture which is about the same size as the diameter of the adjacent portion of the throat 18, so that the washer does not restrict the passage of sound waves through the throat. The ,filters 30 are preferably made of porous bronze material such as is most commonly used as a fuel filter for internal combustion engines and the like. A satisfactory material is a Porex bronze filter, Grade 1 (coarse), which can be purchased from the Delco Moraine Division of General Motors Corporation, Dayton, Ohio. Such a filter 30 is reportedly fabricated by compressing literally thousands of small spheriods of bronze under proper heat conditions until a rigid mass results. .Such a filter is represented in FIG. 3 with slight exaggeration of the particle sizes for the purpose of clarity.

In operation of the loudspeaker 16, the driver unit 20 receives signals through the electrical leads connected to a voice coil (not shown), and sound waves are propagated in a conventional manner by some type of diaphragm or vibratile element. Upon exiting the sound output aperture 21, the

sound waves pass through the plurality of filters 30 and through the throat 18, after which they follow the radially expanding path between the horn walls until they exit the horn mouth 19. In accordance with convention, the driver unit 20 is said to be loaded by the horn 17; that is, the vibratile element must first drive or vibrate the air column contained in the horn, and this air column in turn will vibrate the air in the surrounding listening area. The loudspeaker 16 is thus known as a hem-loaded loudspeaker. The fact that the loudspeaker 16 is also folded does not alter its inherent hom-loaded characteristics, since hem-loaded loudspeakers can be made in many shapes without materially affecting their efficiency. As might be suspected, the number of filters 30 which are selected for a particular embodiment, whether it be two, three, four, etc does have some effect on the response of the loudspeaker 16; but it has been discovered that by using a plurality of coarse filters, the diminution in response is very slight while the protection achieved against explosion is greatly increased.

As shown in FIG. 4, the curve indicated by the numeral 1 represents the response obtained with a loudspeaker 16 having a single filter 30 which is about one-eighth inch thick. The curve 2 represents the same loudspeaker with two similar filters 30 spaced about one-fourth inch apart. Curve 3 and curve 4 depict perfonnance with three and four filters 30, respectively, each spaced about one-fourth inch from the adjacent filter. It will be seen that the response with four filters has almost exactly the same pattern as the response with one filter, and is only slightly lowered in intensity. By wayof contrast, the curve indicated by the numeral 5 reflects the response with a single filter which is physically similar in thickness and shape to the coarse filter 30, but which is fabricated from much smaller particles and which is more tightly compacted. This latter filter can also be purchased from the Delco Moraine Division of General Motors Corporation, and is generally identified as a Porex bronze filter, Grade 4 (fine). The coarse filters will not allow a particle to pass therethrough which has a diameter of 0.0075 inch or larger, while the fine filter will trap all particles in excess of0.()0l inch.

As inferred earlier, the very slight diminution in loudspeaker response makes it practical to use multiple, coarse filters 30, and the protection afiorded thereby makes it possible. To fully explain this, the movement of gases and gassuspended particles must be examined as they both enter and leave the throat 18 of the horn. Further, there are at least two types of hazards to be guarded against in explosion-proof loudspeakers and each must be discussed.

First, it will be assumed that the loudspeaker 16 is placed in a dusty environment, as in a flour mill, a cotton gin, etc. Further, it is assumed that the dust particles suspended in the air will tend to migrate through the filters 30 toward the driver unit 20. Those particles, however, that succeed in passing through the first filter 30 enter what may best be described as a dead space, i.e., a space which lies between two filters and interiorly of a washer 31. This dead space is somewhat analogous to a trap or a settling basin in a simple water purification system, and it promotes the settling of dust particles before they can reach the driver unit 20. When they settle, the dust particles accumulate at the bottom of the dead space and do not tend to clog up a subsequent filter; over a long period of time this is important because it keeps the filters relatively free of dust accumulations and permits relatively uninhibited passage of sound waves through the throat 18. A serial array of three filters 30 will of course be more efficient in this regard than only two such filters, and four will be more efficient than three; but it has been found that two such filters provide excellent protection and will be adequate for most dusty environments.

The second type of environment in which explosion-proof loudspeakers are advantageously employed is one in which the environmental gas itself is explosive, including, for example, methane, hydrogen, etc. Before the discovery described herein, it was believed that one of the fine, grade-4 filters was necessary to offer protection against this type of hazard. It is now known that in such an environment, a plurality of coarse, spaced filters 30 can ofier comparable protection. Each pair of adjacent filters 30 create therebetween relatively stagnant chambers of air that inhibit the ingress of environmental gas into the driver unit 20, and the trapped air between successive filters serves to dilute any potentially explosive gases that over a long period of time may eventually pass through the first filter.

Examining next the performance of the loudspeaker 16 if an explosion does somehow does occur within the driver unit 20, it is found that the serially arranged filters 30 are still highly functional. Being porous, the filters 30 can begin to dissipate the excess pressure resulting from the explosion just as soon as it begins to build up, such that both the resultant rate of pressure buildup and the total pressure experienced in the driver unit 20 are reduced. The possibility of rupture of any of the components of the driver unit 20 is thereby diminished. in the event of an internal explosion, it can be assumed that hot gases will be expelled from the sound output aperture 21 of the driver unit 20 and will proceed down the throat 18 of the horn 17. in this event, the innermost filter 30 which is encountered first will somewhat impede, but will not stop, the onrushing flow of hot gases; but it will stop solid explosion products (such as likely pieces of fractured copper wire from the voice coil, etc. The first-encountered, metallic filter 30 does, however, serve as a thermal sink for the gases; and, as they pass through the filter they begin to lose some of their heat. The captive air held between adjoining filters 30 also serves as a thermal sink, such that eventually the once-buming combustion gases are no longer hot enough to propagate a further explosion when they contact the environmental gases downstream in the air column. It will be recognized that this is the criterion for an explosion-proof device, and the loudspeaker 16 is therefore accurately characterized as being explosion proof.

lt should be noted that the driver unit 20 has not been described anywhere herein as having any unusual characteristics. Indeed, this is a marked advantage of the present invention, i.e., that any conventional compression driver can be combined with any horn in an explosion-proof loudspeaker by merely closing the sound output aperture as described herein with a plurality of serially arranged, spaced filters and, if desired, providing external protection of some type for the driver.

Of further advantage is that the cover member 26 in H6. 2 is removable from the front of the loudspeaker 16. Thus, access to the driver unit 20 is possible from the front, such that the loudspeaker can be permanently mounted without concover member rernovably secured over the first end of said housing means, and having the second end of said housing means secured to said horn means in axial alignment therewith;

driver means disposed within said housing means and secured to the second end of said housing means, said driver means including a sound output aperture axially aligned and in communication with the throat of said horn means;

plural filter means disposed in spaced relationship and serially arranged between said driver means output aperture and said horn means throat, and including a spacer element having an a rture therethrough being disposed to provide a selecte space between each serially arranged pair of said plural filter means; and

securing means having a central aperture and being affixed to said driver means to receive said sound output aperture axially through said central aperture, said securing means being rigidly fastened to said housing means second end to maintain said plural filter means and space? means in fixed position relative to said horn means and throat.

2. A loudspeaker as set forth in claim 1 which is further characterized in that: each of said filter means has a grain density such that it will pass substantially all particles having a dimension of about .0075 inch and smaller.

3. A loudspeaker as set forth in claim 1 which is further characterized in that: each of said filter means is a body consisting of a plurality of sintered bronze spheroids.

4. A loudspeaker as set forth in claim 3 which is further characterized in that: each of said filter means has a thickness of approximately one-eighth inch.

5. A loudspeaker as set forth in claim 3 which is further characterized in that: each of said filter means is spaced from an adjacent filter means by approximately one-quarter inch.

6. A loudspeaker as set forth in claim 1 which is further characterized in that: each of said filter means is selected to have a granular porosity such that plural filter means will trap particles having a diameter of at least .0075 inch. 

